In the formation of integrated circuits (IC), thin films containing metal and metalloid elements are often deposited upon the surface of a semiconductor substrate or wafer. Thin films provide conductive and ohmic contacts in the circuits and between the various devices of an IC. For example, a thin film of a desired metal might be applied to the exposed surface of a contact or via hole in a semiconductor substrate, with the film passing through the inculpative layers of the substrate to provide plugs of conductive material for the purpose of making interconnections across the insulating layers.
One well known process for depositing thin metal films is chemical vapor deposition (CVD), in which a thin film is deposited using chemical reactions between various deposition or reactant gases at the surface of the substrate. In CVD, reactant gases are brought into proximity to a substrate inside a vacuum reaction chamber, and the gases subsequently react at the substrate surface resulting in one or more products which form a film on the exposed substrate surface.
When copper is the metal film deposited onto a substrate by CVD, titanium nitride (TiN) is the common surface underlayer because of its high diffusion barrier strength. When the titanium component, that is, the titanium rich surface layer of TiN, is in its native metal form, a copper film is rapidly deposited on the substrate surface by CVD because the titanium metal provides free electrons needed for this electron transfer reaction. When the titanium component of the TiN is oxidized, or when the substrate surface is nitrogen rich, a copper film does not deposit until a nucleation layer of a copper metal precursor forms on the substrate surface. The lag time while the nucleation layer forms, also called the incubation time, is unproductive because copper cannot deposit on the oxidized substrate surface until the nucleation layer forms.
The probability of copper precursor molecules finding a stable site on the substrate surface on which to deposit is referred to as its "sticking coefficient." The copper sticking coefficient is only about 1% when the substrate surface is TiN, while it increases to about 100% when the substrate surface is covered by a fresh copper film.
A common problem in copper deposition on a substrate by CVD is the so-called "edge effect." The edge effect is defined as occurring when the edge of the substrate gets less copper deposited or no copper deposited compared to the non-edge regions of the substrate. The edge effect is the result of the difference of materials on the susceptor, mainly fresh copper from previous depositions, and the substrate itself, mainly an oxidized form of a metallic layer. The sticking coefficient of the reactant precursor is 100% on the susceptor and much less on the substrate. Therefore the susceptor will consume more reactant than the substrate, thus creating a reactant-depleted zone in the vicinity of the substrate edge. By lack of precursor as well as low sticking coefficient, the edge of the substrate will suffer a longer incubation time. The longer incubation time translates into less deposition of a metal in the vicinity of an edge than in the inner part of the wafer or substrate.
It has been observed that the first substrate processed on a new oxidized susceptor does not exhibit an edge effect. This result has led to the common practice of using a new oxidized susceptor for each deposition of copper. This practice, however, is both inefficient and costly for commercial semiconductor production. Eliminating the edge effect would increase the rate at which a metal film is deposited on a substrate surface edge, which in turn would decrease the incubation time at an edge. The result would be increased efficiency of semiconductor substrate production. A commercially feasible method to eliminate an edge effect in semiconductor substrate production is therefore needed.